At the earlier stage of the development, I thought that it is useful if
lexer-combinators generate error messages, because by doing this,
parser would not need to generate an error message. But now it turned
out that to show an appropriate error message, parser need to generate
according to the context. And almost all the messages from lexer are
discarded. So I added another parameter to lexer-combinator to suppress
error message generation. In the future, we may want to remove messages
completely from lexers, but currently I will keep it. Removing those
unused message generation makes the parsing process faster.
`location::line_num()` function used to be implemented by using
`std::count`, so each time the parser encounters a type mismatch,
`std::count` was called with almost whole file. It decelerates the
parsing process too much, so I decided to add `line_number_` member
variable to `location` and add `advance/retrace/reset` to `location`
in order to modify the position that is pointed.
